While a number of efforts are currently being made to achieve a finer pattern rule in the drive for higher integration and operating speeds in LSI devices, deep-ultraviolet lithography is thought to hold particular promise as the next generation in microfabrication technology.
One technology that has attracted a good deal of attention recently utilizes as the deep UV light source a high-intensity KrF excimer laser and an ArF excimer laser of a shorter wavelength. There is a desire to have a microfabrication technique of finer definition by combining exposure light of shorter wavelength with a resist material having a higher resolution.
In this regard, the recently developed, acid-catalyzed, chemical amplification type resist materials are expected to comply with the deep UV lithography because of their many advantages including high sensitivity, resolution and dry etching resistance. The chemically amplified resist materials include positive working materials that leave the unexposed areas with the exposed areas removed and negative working materials that leave the exposed areas with the unexposed areas removed.
In a chemically amplified positive resist composition to be developed with an alkaline developer, a resin and/or compound in which an alkali-soluble phenol or carboxylic acid is partially or entirely protected with acid labile protective groups (commonly referred to as “acid labile groups”) is catalytically decomposed with the acid generated upon exposure, to generate the phenol or carboxylic acid in exposed areas, whereupon the exposed areas are removed with the alkaline developer. In a similar negative resist composition, a resin and/or compound having an alkali-soluble phenol or carboxylic acid and a compound capable of bonding or crosslinking said resin or compound under the action of an acid (referred to as “acid crosslinker”) are crosslinked with the acid generated upon exposure, to render exposed areas insoluble in an alkaline developer, whereupon unexposed areas are removed with the alkaline developer.
On use of the chemically amplified positive resist composition, a resist film is formed by dissolving a resin having acid labile groups as a binder and a compound capable of generating an acid upon exposure to radiation (referred to as “photoacid generator”) in a solvent, applying the resist solution onto a substrate by a variety of methods, and evaporating off the solvent optionally by heating. The resist film is then exposed to radiation, for example, deep UV through a mask of a predetermined pattern. This is optionally followed by post-exposure baking (PEB) for promoting acid-catalyzed reaction. The exposed resist film is developed with an aqueous alkaline developer for removing the exposed areas of the resist film, obtaining a positive pattern profile. The substrate is then etched by any desired technique. Finally the remaining resist film is removed by dissolution in a remover solution or ashing, leaving the substrate having the desired pattern profile.
The chemically amplified positive resist composition adapted for KrF excimer lasers generally uses a phenolic resin, for example, polyhydroxystyrene in which some or all of the hydrogen atoms of phenolic hydroxyl groups are protected with acid labile protective groups. Iodonium salts, sulfonium salts, bissulfonyldiazomethane compounds, N-sulfonyloxydicarboximide compounds and O-arylsulfonyloxime compounds are typically used as the photoacid generator. If necessary, there are added additives, for example, a dissolution inhibiting or promoting compound in the form of a carboxylic acid and/or phenol derivative having a molecular weight of up to 3,000 in which some or all of the hydrogen atoms of carboxylic acid and/or phenolic hydroxyl groups are protected with acid labile groups, a carboxylic acid compound for improving dissolution characteristics, a basic compound for improving contrast, and a surfactant for improving coating characteristics.
U.S. Pat. No. 6,004,724 (or corresponding JP-A 2002-508774) discloses the photoacid generators in the form of O-alkanesulfonyloxime compounds as shown below. They have a satisfactory sensitivity and resolution and are free of such drawbacks as poor compatibility with resins and low solubility in resist solvents as found with other photoacid generators like sulfonium salts and iodonium salts. They are advantageously used as the photoacid generators in chemically amplified resist compositions, especially chemically amplified positive resist compositions adapted for KrF excimer laser. JP-A 2002-202603 describes a wide spectrum of photoacid generators and discloses that by combining resins having acid labile groups, typically acetal groups, with these photoacid generators, improvements in nano-order edge roughness and the footing of resist pattern on silicon nitride substrates are achievable. JP-A 2003-307840 discloses that by combining resins having acid labile groups, typically acetal groups, with oximesulfonate photoacid generators, improvements in line edge roughness on organic antireflective coatings and PED stability are achievable.

As the requisite pattern size is reduced, however, there arise problems. For example, even the use of resist compositions having these photoacid generators combined with the existing acid labile group-bearing resins encounters the problem of poor resolution. In the step where low reflectance substrates such as organic antireflective coatings cannot be used, for example, ion implantation and other steps where resist patterns are formed on inorganic substrates such as silicon, silicon nitride, titanium nitride, silicon oxide substrates and inorganic nitride films, even the use of resist compositions having these photoacid generators combined with the existing acid labile group-bearing resins encounters the problem that the influence of standing waves by substrate reflection or the influence of contamination from the inorganic substrates prevents formation of satisfactory pattern profiles.
Particularly when different species of materials are present on an inorganic substrate, for example, when plural species of materials having different electric properties, like silicon nitride and silicon, are present within a common plane because of device design considerations, the reflectance or the degree of contamination from the substrate shows local variations. Accordingly, in order to form a pattern from a single resist material, there exists a need for a material which is less sensitive to reflectance or contamination from the substrate.
For resolution improvement, it is a practice to use more acid labile groups, typically more acid labile acetal groups. However, the reduction of pattern size invites a tendency of reducing the thickness of resist film as well. When a phenolic resin having acid labile groups of acetal type is used, the resist surface becomes more dissolvable. The dissolution of the resist pattern at the top raises problems including a top loss that the pattern profile shape is rounded at the top. The remaining resist pattern becomes thinner, making it impossible to secure a film thickness sufficient for the etching or ion implantation step.
When the resist is applied onto inorganic substrates such as SiON substrates which are highly reflective substrates, there arises a problem that standing waves prevent formation of a satisfactory pattern shape. One approach for avoiding these problems is to use resins having acid labile groups of tertiary ether or tertiary ester type which are less labile than the acid labile groups of acetal type (see JP-A 2005-8766). This approach is successful in reducing the top loss, but fails to overcome the problems of micro-pattern rectangularity, footing and standing waves, and is low in resolution.
The photoacid generator for use in resist compositions is required to have a fully high solubility (or compatibility) in resist solvents and resins, good storage stability, non-toxicity, ease of application, pattern profile shape, PED stability, high resolution, wide focal depth, and high sensitivity. The O-arene or alkanesulfonyloxime compound photoacid generators and resist compositions comprising the same do not satisfy all these requirements, especially with regard to the pattern profile shape on inorganic substrates.
In the recent stage when the pattern feature of integrated circuits becomes more miniaturized, more stringent requirements are imposed on the problem of pattern profile shape accompanied with a resist thickness reduction.